This invention relates to a process for leaching noble metals, essentially gold and/or silver, from ores or ore concentrates using an aqueous, alkaline cyanide solution with addition of hydrogen peroxide as oxidizing agent.
It is known that noble metals can be leached with a cyanide-containing solution and, at the same time, converted into soluble cyano complexes. Since the gold in gold ores is mostly present in elemental form, an oxidizing agent is required for the dissolving process. The oxidizing agent used in gold mining is generally atmospheric oxygen. The following equation represents the known reaction: EQU 4 Au+8 CN.sup.- +O.sub.2 +2 H.sub.2 O.fwdarw.4 [Au(CN).sub.2 ].sup.- +4 OH.sup.-
To this end, the finely ground ore is suspended in water, a pH value of from 9 to 12 and preferably from 10 to 11 is adjusted by addition of lime and an aqueous cyanide solution is added. The ore pulp is then stirred for up to 48 hours in one or more cylindrical stirring vessels arranged in the form of a cascade and, at the same time, is gassed with air introduced through nozzles.
However, leaching by agitation, which has been practiced for almost 100 years now, is attended by certain disadvantages; namely:
The oxidation of the noble metal by dissolved oxygen is the speed-determining factor in the dissolution of the noble metal in cyanide-containing solutions. Due to the enormous dimensions of the leaching tanks in use today (up to 3000 m.sup.3) and to the viscosity of the ore pulp, adequate mixing is not often achieved, so that the maximum content of dissolved O.sub.2 determined by the O.sub.2 partial pressure of the air (8-9 ppm O.sub.2 is the saturation limit) is not reached (cf. Example 3a).
The gassing with air means that a more or less large quantity of HCN is always discharged from the ore pulp. As a result, not only is the cyanide demand increased, the safety of the person in charge and the environment are also affected.
The carbon dioxide in the air reduces the pH value of the pulp. As a result, the equilibrium EQU CN.sup.- +H.sub.2 O.revreaction.HCN+OH.sup.-
is displaced towards the free hydrocyanic acid. The result of this is that the lime consumption increases and more hydrocyanic acid is discharged with the air. In addition, calcium carbonate is formed, leading to unwanted deposits on the tank walls, pipes and, in particular, on the active carbon which is often used in the second stage of the process for separating gold. The consequences of such deposits on the carbon are operational disruptions and losses of noble metal.
As a result of the gassing with air, compressor oil can enter the leaching tanks, resulting in smearing of the surface of the active carbon which is used in the CIP process. Operational disruptions and losses of noble metal are again the outcome.
The operating costs of the compressors are a function of the tank height on account of the hydrostatic pressure. Accordingly, the leaching tanks used today, which are up to 20 meters in height, involve increased costs.
In addition to leaching by agitation, so-called heap leaching is used to leach noble metals with a cyanide-containing solution. To this end, large heaps of ore (generally 3 to 10 meters in height) are sprayed with an aqueous cyanide-containing leaching solution having a pH of from 8 to 13. The ore-free leaching solution issuing from the bottom of the heap is circulated, part of the leaching solution being continuously removed from the circuit for the separation of noble metal and replaced by fresh leaching solution.
A serious disadvantage of heap leaching is that the atmospheric oxygen required to oxidize the noble metal in the ore heap has to be introduced into the ore heap by the leaching solution. Since the concentration of dissolved oxygen in the leaching solution decreases to a considerable extent from the top to the bottom of the ore heap, leaching is never complete, above all in the lower regions. This fact is responsible for the generally very low gold yield (40 to 60% of the gold present in the ore) in heap leaching.
In view of these problems, various attempts have already been made to use other oxidizing agents instead of atmospheric oxygen in the leaching of gold, for example permanganates, persulfates, peroxides, ozone, chromates and dichromates, ferricyanides, cyanogen bromide and bromine chloride and also hydrogen peroxide. Although these oxidizing agents are capable of increasing the dissolving rate of the gold by comparison with atmospheric oxygen, they have not yet been able to replace atmospheric oxygen because economic use could not be guaranteed. In addition, some of the oxidizing agents mentioned and/or reaction products emanating therefrom are highly toxic, so that the use of these compounds could not be seriously considered on account of possible environmental pollution and the necessary measures to avoid such pollution.
In environmental terms, hydrogen peroxide, which has also been investigated as an oxygen donor or oxidizing agent, is the most suitable oxidizing agent for replacing atmospheric oxygen, because only water and oxygen and no toxic products are formed in the decomposition of H.sub.2 O.sub.2. However, the industrial use of H.sub.2 O.sub.2 has hitherto been prevented both by its inadequate effectiveness and by the economy factor. The economy factor can be adversely affected inter alia by the fact that, under certain conditions, H.sub.2 O.sub.2 is also capable of oxidizing the cyanide, resulting in an excessive consumption of H.sub.2 O.sub.2 and of cyanide.
Another problem is that H.sub.2 O.sub.2 can inhibit the dissolving process through passivation of the gold surface.
In U.S. Pat. No. 3,826,723, there is described a process for the cyanide leaching of gold and/or silver with addition of hydrogen peroxide. The hydrogen peroxide is shown to be added in the form of stabilized H.sub.2 O.sub.2, preferably as a 50% solution, in a quantity corresponding to 0.3 to 15 g H.sub.2 O.sub.2 per liter leaching solution, as calculated from the data in lines 51 to 53, column 2, of the cited U.S. patent, and 1.2 g/ml for the density of the 50% by weight H.sub.2 O.sub.2. According to the only example, in this cited U.S. patent, gold was leached from an ore in a shorter time and in a higher yield, the leaching solution containing 60 g NaCN and 5 ml stabilized 50% H.sub.2 O.sub.2 per liter, than was possible in conventional leaching with atmospheric oxygen as the oxidizing agent. However, such high cyanide concentration (1 to 600 g NaCN/1 leaching solution according to the U.S. patent) completely jeopardize the economy of the process. This is so because, after leaching and separation of the gold, the leaching solution obtained, which is still rich in cyanide, has to be detoxified for example using H.sub.2 O.sub.2. Recycling of the leaching solution is not possible in practice because disruption of the leaching process through accumulations of other metals could no longer be ruled out.
Comparison tests were carried out in accordance with the process described in the said U.S. Pat. No. 3,826,723, a gold ore being leached and the leaching solution containing sodium cyanide in a concentration which had proved to be suitable in the standard process where air is used for gasing. Comparison Example 1 (hereinafter below) shows that the gold yield remains far below the value obtained in the standard process if at the beginning the leaching solution contains 0.033% by weight sodium cyanide and 0.023% by weight hydrogen peroxide added in the form of 35% by weight aqueous H.sub.2 O.sub.2. Under these conditions, therefore, economic leaching was not possible.
E. L. Day (Canadian Mining Journal, August 1967, pages 55-60) investigated a model system for dissolving a gold foil in leaching solutions containing NaCN and H.sub.2 O.sub.2. According to those investigations, the dissolving rate follows a non-uniform trend as a function of the NaCN and H.sub.2 O.sub.2 concentration of the leaching solution. The maximum dissolving rate was obtained with a leaching solution containing 0.025% NaCN and approximately 0.02% H.sub.2 O.sub.2. However, this model only takes into account the dissolving rate of a gold foil in the first 5 to 30 minutes and does not enable any conclusions to be drawn as to the gold yield, the demand for sodium cyanide and hydrogen peroxide and the dissolving rate of gold from gold ores. Although a leaching solution of which the cyanide and H.sub.2 O.sub.2 concentration was in the optimal range of the model system was used in the Comparison Example, as explained above, the process proved to be impracticable in economic terms.